This relates generally to imaging devices, and more particularly, to imaging devices having analog-digital converters (ADCs).
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels arranged in pixel rows and pixel columns. Circuitry is commonly coupled to each pixel column for reading out analog image signals from the image pixels. Analog-to-digital converters are used to convert the analog image signals to corresponding digital image data.
The ADCs are often single-slope ADCs having an integrator and a comparator that provide a digital output. During an integration period, the integrator is used to generate a ramp waveform that is received at a first input of the comparator. A second input of the comparator receives an analog input. The ramp waveform is compared with the analog input until the two signals are equal (e.g., when the comparator changes outputs or “inverts”). The integrator then resets and repeats the ramping behavior, and the comparator repeats the comparison process. Digital values corresponding to every time the comparator inverts are recorded. The digital values correspond to digital data that is outputted as the digital output of the ADC.
In an exemplary configuration, each column of the array of image pixels is coupled to a dedicated ADC (e.g., a dedicated single-slope ADC) for the respective column. However, during the operation of the ADC, comparator inversions and other power consuming events may be concentrated at a given time (e.g., the comparators within respective ADCs for all columns invert at the same time) leading to power line fluctuations (e.g., power spikes, current surges). The concentrated timing of comparator inversions may consequently cause counting jitters, power supply noise, linearity error, and other undesirable effects.
It would therefore be desirable to be able to provide imaging devices with improved analog-to-digital converter circuitry.